1. Field of the Invention
The present invention relates to a thin-film magnetic head used in magnetic recording and/or reproducing apparatus and more particularly to a magnetoresistive head.
2. Description of the Prior Art
Recently, to cope with the trend of higher recording density, higher data transfer speed and multiple channels in magnetic recording and/or reproducing apparatus, the thin-film magnetic head is now being used. In particular, in a magnetic recording and/or reproducing apparatus in which the recording medium runs at a low speed, a magnetoresistive head whose output during reproduction does not depend on the speed is used.
A magnetoresistive head of a simplest structure has already been disclosed, for example, in the published paper, "A Magnetoresistive Readout Transducer," R. P. Hunt, IEEE Trans. Magn. Vol. 1, MAG-7, No. 1 (March 1971), pp.150-154. In this magnetoresistive head, a magnetoresistive element composed of a Ni-Fe alloy thin film with a film thickness of hundreds of Angstroms is disposed vertically or horizontally on the recording medium, and a pair of electrodes are disposed at both ends of this magnetoresistive element. For signal detection, the resistance change of the magnetoresistive element due to the signal magnetic field generated by the recording medium is converted into a voltage change by the constant current in the magnetoresistive element flowing through the electrodes. However, in the magnetoresistive head in which the magnetoresistive element is disposed horizontally, since its reproduction resolution depends on the geometric shape of the magnetoresistive element, a signal recorded at a high density of a recording wavelength of several micrometers or less cannot be reproduced. Or, in the magnetoresistive head in which the magnetoresistive element is disposed vertically, since the intensity of the signal magnetic field from the recording medium is attenuated in accordance with an exponential function with respect to the distance from the magnetoresistive element to the recording medium, and the signal magnetic field becomes smaller generally as the recording wavelength becomes shorter; therefore, reproduction sensitivity is particularly poor in the short wavelength region of a recording wavelength of several micrometers or less. Furthermore, in either magnetoresistive head, the wear of the magnetoresistive element is inevitable because it is kept in contact with the recording medium.
A magnetoresistive head intended to solve the above problems of wear and the lowering of the reproduction capability in the short wavelength region has been already disclosed, for example, in the Japanese Laid-open Patent No. 52-9413. This magnetoresistive head comprises a bias conductor made of a conductive material for applying a bias magnetic field to a magnetoresistive element; the magnetoresistive element; a pair of electrodes for passing a driving current to the magnetoresistive element, and two magnetic yokes, that is, a front yoke and a back yoke, which are sequentially formed on a magnetic substrate by way of an insulation layer such as SiO.sub.2. The front yoke and back yoke are disposed so that one end of each yoke overlaps one of thereof overlap the ends of the magnetoresistive element. Between the front yoke and the magnetic substrate, a magnetic gap is formed in order to lead the recording magnetic field generated from the recording medium into the magnetic head.
At this time, mainly thin-film forming technology such as vacuum deposition and sputtering, or fine pattern technology such as photolithography is employed to produce the magnetic heads.
This magnetoresistive head possesses advantages in that the magnetoresistive element is not worn because the magnetoresistive element does not directly contact the recording medium, and in that signals with a recording wavelength of 1 .mu.m or less are reproduced by setting the magnetic gap at 1 .mu.m or less.
Such a conventional magnetoresistive head is designed generally so that the both ends of the magnetoresistive element overlap the front yoke and back yoke by the same overlapping amount.
In the fabrication of such a magnetoresistive head, however, the position of the magnetoresistive element tends to be shifted longitudinally due to various causes including pattern matching precision in the process of photolithography. Accordingly, when a plurality of magnetic heads are fabricated, their reproduction output varies greatly. This is a fatal problem for fabricating a multi-channel magnetoresistive head.